1. Field
The invention relates to a joint prosthesis with a head cap articulated in the manner of a ball joint via a collar piece on a base piece for anchoring in the bone.
2. Background Information
From EP-A-0 663 193 a hip joint socket is known which is held by a holding ring in a supporting shell on an anchoring plate fixed on the bone. In order to prevent any rotation of the hip joint socket in the supporting shell, penetration elements are provided in the supporting shell. If the hip joint socket is now pressed into the supporting shell by a setting instrument with the application of force, the penetration elements penetrate the relatively soft plastic material of the hip joint socket. The holding ring is then brought into the correct position to hold the hip joint socket in the supporting shell.
From WO 99/34756 a shoulder prosthesis is known wherein a collar piece is articulated in a hemispherical recess in the shaft piece, the collar piece being pivotable therein in the manner of a ball joint. The collar piece has a hemispherical articulation surface and a conical surface which is eccentric in relation to an axis through the ball center of said articulation surface, said conical surface being intended for the fitting of a joint cap. The collar piece has a bore which is open from the cap side and which has a hemispherical base. A screw with a spherical head introduced into the bore and adapted to be screwed into the shaft piece through an opening in the base of the bore, co-operates with the base. The spherical surfaces of the hemispherical recess in the shaft piece, of the articulation surface on the shaft piece, of the base of the bore and of the screwhead must have the same center point. In addition, each pair of co-operating hollow and solid spherical surfaces must be made very accurately and have the same radius. Minimal deviations from the ideal measurements have the effect that the collar piece cannot be connected sufficiently firmly to the shaft piece in order to reliably prevent unintentional pivoting of the collar piece relatively to the shaft piece during the use of the joint.
From EP-A-0 712 617 a humeral head prosthesis is known wherein an articulation ball connected to a head cap via a shank is articulated on a shaft piece in a cavity with a hollow spherical base. To fix the articulation ball in the cavity, one or more grub screws are provided which can be screwed through the shaft piece against the articulation ball. In one exemplified embodiment, the articulation ball, which is of cut-open C-shape, is pressed together by the grub screw in order to clamp therein the shank which fits in a central bore in the articulation ball. In another exemplified embodiment, a grub screw is provided which can be screwed along the shank axis through the articulation ball and against the cavity base. With this screw the articulation ball is pressed against the opening of the cavity, which opening in this exemplified embodiment has a smaller radius than the spherical radius of the cavity and the articulation ball.